Spun out as future work from https://bugzilla.mozilla.org/show_bug.cgi?id=1232205#c33 and previous comments:
We would like the test runner to have the option of not just scanning a file for directives in its header (ie, "|jit-test|" ...) but for it to recognize a per-directory file containing such directives, a strawman name for that file is directives.txt. The meaning of the file would be that the directives in the file would be merged with the directives in each file in the directory.
(I'm on the fence regarding the file applying to subdirectories, I'm thinking probably not.)

Created attachment 8808575[details][diff][review]bug1277770-jit-test-directives-from-file.patch
Looks for directives.txt in the directory of the test case, and extracts a jit-test directive out of that if there's one. It is then catenated with whatever's in the test file. The directive is cached in the test runner, though I don't know if that really is necessary.